The present invention pertains to an apparatus for monitoring the density, and thus the mass quantity of a fluid within a closed container. The present invention further pertains to a method of filling a container with a fluid to a predetermined mass quantity. Additionally, the present invention relates to an improved air bag system for an automotive vehicle, including a sensor and an indicator for providing an indication of leakage of the propellant gas with which the air bag is to be filled.
The density of a fluid in a closed container of fixed volume is proportional to the mass quantity of the fluid in the container. Therefore, monitoring the density of the fluid permits determination of the mass quantity of the fluid in the fixed volume. Monitoring of the density of a fluid which is under a high pressure in a closed container preferably is done without penetrating the closed container, so as to avoid leakage of the fluid from the container.
In many applications, the monitoring need only provide an indication of whether the density, and thus the mass quantity, are sufficiently high; i.e., an indication of whether the density or the mass quantity is above or below a predetermined value. In such applications, use of a two state monitoring apparatus is appropriate. In other applications, a more precise monitoring might be desired, providing a more accurate indication of the fluid density and mass quantity.
Many vehicular air bag systems include a canister of propellant gas for inflating the air bag if an impact is sensed. The mass quantity of this propellant gas should be monitored to assure that a sufficient mass quantity of the gas is present to inflate the air bag properly in the event an impact is sensed. Monitoring of the propellant gas density, to assure that the propellant gas maintains at least a specified density, is one way of monitoring the mass quantity of the gas. A drop in the propellant gas density can indicate a drop in the mass quantity of the gas within its container, which might be due to leakage of the gas from the container. Such leakage can result in the vehicle air bag system no longer having enough of the gas, and so result in the air bag not inflating satisfactorily in the event of an impact. Since many automotive vehicles are used for several years without activation of the air bag system, the propellant gas may leak over the life of the vehicle and the air bag system. Thus, monitoring the density of the propellant gas is desirable to permit detection of gas leakage to an inappropriate level.
A container of pressurized fluid which is expected to have a lengthy life is preferably filled with a mass quantity of the fluid exceeding the lowest acceptable mass quantity so that, despite minor leakage, the container will maintain at least the acceptable mass quantity of the fluid over its expected life, or at least over a significant portion of that life. The mass quantity can be monitored by monitoring the density of the fluid within the container, and so it is desirable to be able to monitor two fluid density levels within the container of fluid. The first density level is the density level corresponding with the lowest acceptable mass quantity of the fluid in the container, and the second density level is the density level corresponding with the mass quantity to which the container of fluid is to be filled at the time it is manufactured or refilled. In other applications, monitoring of several density levels may be desirable.